Cdk/Cyclin complexes drive progression through the eukaryotic cell cycle. Entry into mitosis is triggered by Cdk1 (Cdc2)/Cyclin B complexes, while M phase exit requires Cyclin B degradation and dephosphorylation of mitotic phosphoproteins. The destruction of Cyclin B at M phase exit is driven by the APC (anaphase promoting complex), a multi-protein E3 ubiquitin ligase, which ubiquitylates Cyclin B and promotes its proteasomal degradation. Multiple factors cooperate to prevent premature activation and ensure timely inactivation of the APC. In vertebrate eggs, which are arrested in M phase for prolonged periods of time, the APC is restrained by an activity known as Cytostatic Factor (CSF). Recent findings in both Xenopus and mouse eggs have strongly implicated Emi2 protein as a critical component of CSF activity, responsible for direct inhibition of the APC. We do not understand precisely how Emi2 effects APC inhibition, though we have recently found that Emi2 appears to act catalytically, rather than stoichiometrically, as is generally believed. Moreover, our data indicate that Emi2 function is inhibited, at least in part, through direct Cdc2-mediated phosphorylation, though the nature of the phosphatase complexes that counter this to activate Emi2 function are unknown. The aims of this proposal are to elucidate the mechanism(s) of APC inhibition by Emi2 and to determine how Emi2 is regulated by CSF. This understanding is critical in that inappropriate APC activation at meiotic exit can lead to errors in chromosome segregation and severe developmental abnormalities underlying a range of birth defects.